I am going to order some K9 Advantix and Athena is right at the dividing point between does 21-55 (she is 56lbs) so I am going to give her the lower dose. But Sal is 63lbs, I am leaning towards giving him the lower dose because I am a little worried about the next dose being good for dogs twice (or more) his size as the dose says 55lbs and up. Meaning that it would have a big enough dose for a 200lb Eng. Mastiff Am I just over thinking this? If I give Sal the 22-55 dose will I be wasting my money?